In the manufacture of integrated circuits (ICs) it is typical that a number of ICs are formed simultaneously on a semiconductor substrate such as a silicon (or other suitable material) wafer, which is generally circular in shape. The diameters of silicon wafers have steadily increased over the years, ranging from approximately 25 mm to the present-day state-of-the-art 300 mm. It is anticipated that 450 mm wafers will come on line in the years ahead. Older sized silicon wafers, such as 100, 150, and 200 mm, remain in production for numerous legacy products. Furthermore, ICs and semiconductors based on non-silicon and/or composite materials, such as GaAs, AlGaAS, SiGe, etc., are typically fabricated on wafers in the 50 to 200 mm diameter range. The typical thickness of a wafer ranges from approximately one-half to one mm, depending, in part, upon its diameter. Generally speaking the number of ICs of a given complexity that can be formed on a single wafer increases with the square of its diameter. Further, as IC fabrication process technology advances, the physical size of the individual transistors, conductors, and other features that comprise an IC shrink, enabling still further increases in the number of ICs that may be fabricated on a single wafer. As the number of ICs that can be produced on a wafer increases, the cost per IC correspondingly decreases. Consequently, larger diameter wafers provide opportunities for economic benefit.
In an IC fabrication process a wafer may typically go through a number of processing steps on a number of different machines. Wafers are typically moved from one machine to the next by automated equipment. Automation devices also place each wafer in turn within each machine for the respective processing step to be performed on it. In order that a wafer be properly aligned with and located in a machine, it is common that a physical indexing feature is included in each wafer. This feature may be formed as the crystal from which the wafer is cut is formed and may be indicative of its crystalline orientation. As illustrated in FIG. 1A it is usual in 200 and 300 mm diameter silicon wafers that the indexing feature is a small notch 11 in the periphery of the wafer 10. In smaller-sized silicon wafers (as well as certain 200 mm wafers) the indexing feature, as illustrated in FIG. 1B, is typically a flat segment 13 formed in the perimeter of the wafer 12. It should be noted that FIGS. 1A and 1B are not drawn to scale; in particular, for purposes of illustration, the indexing features are shown to be much larger than they actually may be. For example, notch 11 in FIG. 1A may actually be less than one or two square millimeters in size.
In one or more stages of the fabrication process, a wafer may be coated with a film of photoresist. The photoresist-coated wafer may then be exposed to electromagnetic radiation of an appropriate wavelength. A mask detailing selected geometric features of the ICs being fabricated may be placed between the source of the radiation and the wafer. In this way areas of photo resist will be exposed and the remaining areas will be non-exposed according to the geometric pattern defined by the mask. Photoresist is of two general types, positive and negative. When positive photoresist is used, exposure to electromagnetic radiation of an appropriate wavelength enables the exposed resist to be washed away with an appropriate solvent while non-exposed areas remain. The areas of the wafer thus exposed may then be uniformly treated in the next step of the fabrication process. When negative photoresist is used, the non-exposed areas may be readily washed away while the exposed areas remain. Over time, positive photoresist has become the most commonly used type. Typically electromagnetic radiation with a wavelength in the neighborhood of 400 nm or ultraviolet region is typically required for its exposure; however, other types of positive resist requiring other wavelengths are known.
A common method of applying photoresist to a wafer is spin coating. In this method the wafer is held on a chuck, often by vacuum means, so that its surface to be coated is exposed, horizontal, and facing upwards. The chuck is rotated about a central vertical axis causing the wafer to spin. An appropriate amount of liquid photoresist is deposited at or near the center of the spinning wafer. Centrifugal force then causes the resist to be uniformly distributed over the surface of the wafer. However, a bead of photoresist will usually form at the edge of the wafer. This is illustrated in FIGS. 2A and 2B. FIG. 2C is provided to illustrate the same wafer after the edge bead has been removed. It is to be emphasized that FIGS. 2A-2C are for illustrative purposes and not drawn to scale. FIG. 2A is a perspective view of a wafer 20 that has a coating 21 of photoresist; and FIG. 2B is a cross section of the wafer taken on a diameter of the wafer. Edge bead 22 of resist is seen at the edge of the wafer 20. As shown edge bead 22 rises higher than coating 21 on the surface of wafer 20, and it may extend over the edge, downwards on the cylindrical outer periphery 23 of wafer 20. Typically, photoresist coating 21 may be approximately 25 microns thick across the surface of wafer 20, except at the edge where the edge bead 22 has formed. Edge bead 22 may have a peak height above the wafer of 50 to 75 microns (that is, 25 to 50 microns above the surface of coating 21), and it may extend a distance of typically one to as much as two mm inwards from the edge 23. As shown in FIG. 2B, “D” defines the diameter of wafer 20, and “W” defines the maximum width on the wafer surface that edge bead 22 could occupy. (The fact that the ratio of W to D is typically less that 0.01 indicates how significantly out of scale the Figures are.)
For a number of well-known reasons, it is generally desirable to remove edge bead 22 before proceeding to the next step in the manufacturing process, and a number of methods exist in the prior art for doing so. It is often preferred to entirely remove the photoresist at the wafer's edge, thus exposing the wafer's surface 24a as illustrated in FIG. 2C, which is a cross sectional illustration of the wafer 20 of FIGS. 2A and 2B after edge bead 22 has been removed. Note that the removal of resist has occurred for a distance W inwards from the edge of wafer 20 as well as along the outer periphery 23 of wafer 20. A number of techniques are known for edge-bead removal. For example, mechanical methods, such as grinding, are known. Such methods may also be used at other steps of the fabrication process; for example, for removing metallic edge beads created when metallization process steps are performed. Solvent-based edge-bead removal methods, applicable to either positive or negative photoresist, are also known, such as described, for example, in U.S. Pat. No. 6,565,920. In such methods, it is typical to rotate the wafer so that a nozzle-directed stream of an appropriate solvent is uniformly applied to its circumferential edge, thus washing the edge bead away and exposing the bare, underlying surface of the wafer.
For the removal of positive photoresist edge beads, techniques involving the exposure of the edge bead to electromagnetic radiation of an appropriate wavelength to activate the photoresist followed by washing away the residue. In such techniques, care must be taken to sufficiently irradiate the photoresist in the region of the edge bead while not affecting the photoresist that covers the rest of the wafer.
One prior art approach is to use a large mercury-vapor lamp as a source of electromagnetic radiation of the required wavelength. Mercury vapor lamps having strong energy emission characteristics in the neighborhood of 400 nm are compatible with typical positive photoresists and have thus been used for this purpose.
Reflective housings, shadow masks, etc. may be used to apply the lamp's radiation to the edge region of a wafer without affecting the photoresist-coated interior region. However, a significant portion of the lamp's energy may be either absorbed or reflected by interior of the housing, an opaque shadow mask or otherwise wasted in the process, resulting in unfavorable efficiency. The wafer is held in a stationary position as the radiation is applied uniformly to its entire edge. The large mercury-vapor lamp is positioned so that the light that is emitted from the mercury-vapor lamp is emitted from a position that is centered relative to the shadow mask. The goal of this positioning is so that the light that illuminates the wafer is uniform on the wafer. In other words, it is desired that all portions of the wafer illuminated by the lamp are illuminated uniformly. By centering the emission of light relative to the mask (situated between the light emission and the wafer), the portion of the wafer that is illuminated is illuminated uniformly. Power requirements for these types of systems may disadvantageously range from several hundred watts to as much as 3000 to 5000 watts, depending in part upon the diameter of the wafer and the type of resist. In a production setting the mercury vapor lamps typical have relatively short useful lifetimes and can be costly to replace. Typically, the time required to thoroughly activate the photoresist with this technique is approximately two to three minutes.
A second prior art approach to the removal of a positive photoresist edge bead involves focusing a small, focused beam of radiation of the appropriate wavelength onto the edge of the wafer. The wafer is held on a turntable and slowly rotated so the entire edge passes the beam of radiation. With this approach the power requirements for the radiation source are much smaller and less power is wasted. However, the process may take several times longer.